That's a Palm database file, Ku'u. Unfortunately (for you) there's a version of Acrobat Reader for Palm so I never had to do pdf's. I have a program to convert .lit (Microsoft ebooks) to pdb but it doesn't support pdf.
If what you have so far doesn't get you going you might surf some of the Palm forums for help. I use a Blackberry now but my ebook program still uses pdb files.